“I do not like vegetables”

“Yes but it is good for your health”

We all probably had to deal with our kids refusing to eat vegetables. We tried to convince them that it is good for their health because it brings vitamins. The proper explanation is actually more complex than only vitamins but it requires to have a good understanding of the digestion physiology.

For years, we have been told that most of the digestion occurs in the stomach and the beginning of the small intestine and then nutrients are absorbed in the rest of the intestine tract. But it is now confirmed that a major part of the digestion takes place as well in the colon and it is important to understand the differences between these two different processes.

First, it is critical to remember that in adult animals, the supply of energy comes at 75% as result of the digestion occurring in the stomach and up to 25% is the result of the fermentation happening in the colon. Therefore, nutritionists who are looking at optimizing the cost of energy supplied to monogastrics needs to pay attention of what is happening in the colon.

Direct vs Indirect digestion

In the stomach and duodenum, the digestion is direct. The nutrients provided through the diet are cut in small pieces by the enzymes and then transported to the liver where they will used as such. It is a direct utilization of the nutrient.

The nutrients that pass the small intestine and arrive in the colon, will encounter a large population of bacteria who will use them as source of energy to grow and multiply. As results, these bacteria will produce acetate, propionate or butyrate. These acids will pass in the blood and go to the liver who they will be transformed into glucose. It is an indirect process where the diets feed the bacteria who will in return feed the animal.

Enzymatic versus Fermentation

The mechanism of utilization between digestion in the upper part of the gut and fermentation in the lower part is sensibly different. The digestion is enzymatic. It will use endogenous enzymes produced by the animals or exogenous enzymes produced by some probiotics or additives supplied in the diet. The fermentation is actually a process involving bacteria. The microbial composition of the gut microbiota varies across the digestive tract. In the stomach and small intestine, relatively few species of bacteria are generally present. The colon, in contrast, contains the highest microbial density recorded in any habitat on Earthwith up to 1011 cells per gram of intestinal content. These bacteria represent between 300 and 1000 different species. However, 99% of the bacteria come from about 30 or 40 species. As a consequence of their abundance in the intestine, bacteria also make up to 60% of the dry mass of feces.

Over 99% of the bacteria in the gut are anaerobes. It is estimated that these gut flora have around a hundred times as many genes in total as there are in the human genome. Many species in the gut have not been much studied outside of their hosts because most cannot be cultured.

The balance between all these populations is very fragile. Some bacteria are strict anaerobic, some others are facultative aerobic. Some bacteria are cellulolytic (ferment fibers) and some others are proteolytic (they ferment proteins). To optimize the capacity of the colon to supply energy to the liver through production of acetate, propionate and butyrate, we need to ensure a healthy balance of all these populations. That depends on the nutrients that will reach the colon. If we provide too much undigested protein, that will develop the proteolytic population (E.Coli, Clostridium). If we do not provide enough fermentable fibres, that will depress cellulolytic population as firmicutes. If stress is too high, that will lead to the passage of oxygen from the blood to the gut and will depress the strict anaerobic bacteria. Nutrition is a key driver to maintain proper gut flora balance and health. It is important to remember that gut inflammation is one of the major consumer of energy and it needs to be avoided at maximum if we want to promote better animal growth.

Digestibility vs Fibre

For the last 50 years, the focus of nutritionists was on the upper part of the tract concentrating on the enzymatic digestion. We wanted to be sure that all the nutrients supplied in the Swine and Poultry diets were as digestible as possible meaning that they should get into the blood as early as possible in the tract. We wanted to minimize any nutrients that would not be absorbed in the duodenum or ileum. That would be seen as waste. But we never consider that we were then depraving the 1011 bacteria in the colon from any nutrient.